This invention lies in the area of systems and methods for making level measurements on homogeneous materials contained by a vessel and, more particularly, two channel systems for providing a compensated level system in an industrial environment.
There are a great variety of industrial applications where it is necessary to have an accurate and reliable material level measuring system for use in industrial vessels. The level measuring system must be adaptable to work reliably in a variety of arduous environments such as exist within such vessels. Accordingly, a premium is placed upon having system designs which provide minimum complexity and maximum reliability and, of course, efficiency in terms of system cost. A widely used basic measuring system is the capacitive system where an electrode is used in combination with the vessel walls, the vertical probe rod or cylinder of the electrode comprising a first electrode and the vessel walls comprising a second or return electrode. This system is driven by a remote electronic unit attached thereto, which provides A.C. driving signals for deriving output signals as a function of the effective capacitance value of the probe. Since this capacitance value reflects the material level, the signal reflects such level. Such capacitive systems require minimum hardware within the vessel. However, they do not inherently compensate for errors due to changes in the dielectric constant of the material or to certain changes in the interface electrodes which form the electrical interface to the material. Additionally, for cases where the measured material within the vessel is non-conductive, the probe to vessel electrode geometry plays a significant role in the functioning of the system. The first step to providing means for compensating systems is the use of a second, or reference probe which is maintained submerged in the material contained by the vessel. Such two probe capacitive level measuring systems achieve a certain independence from variations in the dielectric constant of the material. However, the simple provision of a second reference probe does not eliminate error from the system, and it remains necessary to provide further compensation by adjustment of the reference signal in order to obtain substantially complete independence from variations in the material dielectric constant.
In the following description, the material "level" is defined as a uncompensated level, and is the measured level obtained by the level probe, which is affected by the electrical characteristics of the material. The "compensated level" is the actual level which is a corrected value of the measured level. The compensated level is obtained by separately measuring the electrical characteristics with a reference probe, and using this measured value to compensate the level signal from the level probe.